1. Field of the Invention
This invention pertains generally to circuit interrupters and, more particularly, to such circuit interrupters structured to trip open separable contacts responsive to arc fault, ground fault and/or overvoltage conditions.
2. Background Information
Circuit interrupters include, for example, circuit breakers, contactors, motor starters, motor controllers, other load controllers and receptacles having a trip mechanism. Circuit breakers are generally mature and well known in the art. Examples of circuit breakers are disclosed in U.S. Pat. Nos. 5,260,676; and 5,293,522.
Circuit breakers are used to protect electrical circuitry from damage due to an overcurrent condition, such as an overload condition or a relatively high level short circuit or fault condition. In small circuit breakers, commonly referred to as miniature circuit breakers, used for residential and light commercial applications, such protection is typically provided by a thermal-magnetic trip device. This trip device includes a bimetal which is heated and bends in response to a persistent overcurrent condition. The bimetal, in turn, unlatches a spring powered operating mechanism which opens the separable contacts of the circuit breaker to interrupt current flow in the protected power system. An armature, which is attracted by the sizable magnetic forces generated by a short circuit or fault, also unlatches, or trips, the operating mechanism.
In many applications, the miniature circuit breaker also provides ground fault protection. Typically, an electronic circuit detects leakage of current to ground and generates a ground fault trip signal. This trip signal energizes a shunt trip solenoid, which unlatches the operating mechanism, typically through actuation of the thermal-magnetic trip device.
A common type of ground fault detection circuit is the dormant oscillator detector including first and second sensor coils. The line and neutral conductors of the protected circuit pass through the first sensor coil. The output of this coil is applied through a coupling capacitor to an operational amplifier followed by a window comparator having two reference values. A line-to-ground fault causes the magnitude of the amplified signal to exceed the magnitude of the reference values and, thus, generates a trip signal. At least the neutral conductor of the protected circuit passes through the second sensor coil. A neutral-to-ground fault couples the two detector coils that cause the amplifier to oscillate, thereby resulting in the generation of the trip signal. See, for example, U.S. Pat. Nos. 5,260,676; and 5,293,522.
Recently, there has been considerable interest in also providing protection against arc faults. Arc faults are intermittent high impedance faults which can be caused, for instance, by worn insulation between adjacent conductors, by exposed ends between broken conductors, and in other situations where conducting elements at different potentials are in close proximity. Because of their intermittent and high impedance nature, arc faults do not generate currents of either sufficient instantaneous magnitude or sufficient average heating or RMS current value large enough to trip the conventional circuit interrupter. Even so, the arcs can cause damage or start a fire if they occur near combustible material. It is not practical to simply lower the pick-up currents on conventional circuit breakers, as there are many typical loads which draw similar currents and would, therefore, cause nuisance trips. Consequently, separate electrical circuits have been developed for responding to arc faults. See, for example, U.S. Pat. Nos. 5,224,006; and 5,691,869.
For example, an arc fault circuit interrupter (AFCI) is a device intended to mitigate the effects of arc faults by functioning to deenergize an electrical circuit when an arc fault is detected. Known AFCIs are packaged as miniature circuit breakers. They are installed in panelboards, just as standard circuit breakers, and thus provide protection of the complete branch circuit wiring from panelboard to outlet. Additionally, they provide protection of appliance and extension cords against insulation failure related arcing events. While not required by the Underwriters Laboratory (UL) AFCI Standard UL1699, such AFCIs also provide low level ground fault protection, typically set at 50 mA peak. This protection, in combination with arc fault protection, has been shown by UL to be effective in protecting against high resistance connections, such as can develop at connections on electrical wiring devices, such as twist-on wire connectors, receptacles, wall switches or light fixtures, that can ultimately lead to an insulation failure. Neither arc fault nor ground fault alone was shown to provide optimum protection; both are required. Arc fault protection is uniquely effective against a line-to-neutral fault, while ground fault protection is uniquely effective against a neutral-to-ground fault. Both are effective against a line-to-ground fault.
Today, ground fault protection of a ground fault circuit interrupter (GFCI) is required, for example and without limitation, for residential bathrooms, garages, kitchens and outside receptacles. These locations normally do not involve surge protecting power strips or products like computers that sometimes have built-in surge protection. In contrast, AFCIs are used, for example and without limitation, in home bedrooms, studies and other living areas where surge protection is common.
U.S. Pat. No. 6,707,651 and U.S. Patent Application Pub. No. 2006/0018059 disclose a trip unit including a trip logic circuit that is incorporated in a bipolar arc fault/ground fault Application Specific Integrated Circuit (ASIC). The ASIC inexpensively provides suitable gate current in response to one of two trip requests (e.g., arc fault trip and ground fault trip) when a triac is in an OFF state with a suitable supporting voltage and when the ASIC is suitably powered. The trip unit operates with a circuit interrupter, such as an arc fault or ground fault circuit breaker.
Although the ASIC provides a reliable and relatively low cost circuit to trip the arc fault or ground fault circuit breaker, certain nuisance trips may occur. For example, utilities routinely switch capacitors to adjust power factor and, more often, to adjust line voltage in rural areas. The line series source inductance combined with a shunt capacitor yields a series resonant circuit, such that voltage with the capacitor can be slightly larger than that without the capacitor. Unfortunately, when the capacitor is switched in, a decaying sinusoidal transient voltage can occur. This transient voltage typically lasts only a few milliseconds, but can have a peak value nearly twice the normal line voltage peak. If a surge protector, such as those used in conventional power strips (e.g., without limitation, power strips used to protect home electronic equipment, such as televisions or computers), is located on the branch circuit being protected, then the normal peak voltage limiting action of the corresponding power strip MOV(s) (metal oxide varistors) results in a relatively high, but relatively very short, ground fault current. The MOV functions by clamping line-to-ground overvoltage transients. The MOV clamping process can produce a relatively very large current transient (measured in amperes), but a typically relatively very short (e.g., about 100 uS) ground current transient. If the power strip is located on an AFCI protected branch circuit, then the AFCI will (ground fault) trip as the result of such a transient.
The ASIC ground fault trip circuit includes a differential current transformer, an amplifier and a window comparator. The current output of the differential current transformer is input to the amplifier. The output of the amplifier is input to the window comparator. Whenever the magnitude of the amplifier output exceeds the high or low limits of the window comparator, a ground fault trip request is immediately generated with no time delay. Here, there is a problem due to the lack of a time delay.
The ASIC and the corresponding AFCI are designed for nominal 120 VAC operation with a tolerance of +/− 10%. It is possible, for example, during home construction with temporary power, and even after the home is completed and occupied, that an extreme overvoltage condition can develop due to a loss of a service input neutral connection in a 3-wire 120/240 power system or a 3/4-wire 120/208 VAC power system. Without a neutral to establish a midpoint for these power systems, a phase-to-neutral voltage can reach an extreme and damaging value. While the ASIC and the corresponding AFCI components can survive this condition on a temporary basis, certain parts will eventually fail because of overheating.
The ASIC and the corresponding AFCI may trip due to certain loads with a relatively high current inrush. These loads include certain power tools such as “chop saws”, compressors and a number of relatively new and high technology variable speed vacuum sweepers. These loads can be characterized as having a relatively large current with a lagging power factor (unlike, for example, a lamp) during the starting inrush period. Hence, it is desirable to minimize or eliminate tripping resulting from such loads.
The ASIC includes a trigger circuit that generates a single pulse each alternating current line cycle whenever a test button is pushed. This pulse is sent into the arc detection circuitry such that if this circuitry is working correctly, a trip will occur as is required by UL 1699. A recognized weakness of the test button is that determination of health of the AFCI requires pushing the test button, which causes interruption of the branch circuit voltage if the circuit interrupter is healthy. As such, for example, clocks, VCRs and other loads need to be manually reset.
UL943 (Underwriters Laboratory's Ground Fault Circuit Interrupter Standard) is being changed to address this issue. It is believed that one of two approaches will be required. The first approach requires that if the circuit interrupter fails, then the test button test must be “locked out” and not allowed to be reset and to supply power. However, this is not practical for a circuit breaker. The second approach requires a “visible” indication of a failure, which allows the state of the circuit interrupter to be determined without pushing the test button.
There is room for improvement in circuit interrupters structured to trip open separable contacts responsive to arc fault, ground fault and/or overvoltage conditions.
Furthermore, there is also room for improvement in the power dissipation of a circuit interrupter including an ASIC.